Api Rate Abuse in Aspnet with Openid Connect

Rate abuse in ASP.NET APIs that use OpenID Connect for identity can occur when an authenticated identity is leveraged to bypass IP-based limits or when token issuance and introspection endpoints are not included in rate-limiting strategies. OpenID Connect introduces flows such as authorization code, refresh tokens, and token revocation; each can be targeted to exhaust server resources or to amplify abuse through authenticated channels.

Consider an ASP.NET Core Web API configured with AddAuthentication and JWT Bearer options. If rate limiting is applied only to unauthenticated paths or is scoped to IP addresses, authenticated requests carrying a valid access token may escape throttling. For example, an attacker who obtains a valid token (via phishing or token leakage) can perform rapid token introspection or revocation calls, consuming server-side validation and database resources tied to the OpenID Connect provider.

In hybrid flows where ASP.NET handles both identity and API endpoints, inconsistent middleware ordering can create gaps. If the OpenID Connect middleware runs after rate-limiting middleware, authorization requests may not be accounted for correctly. This allows an authenticated user to issue repeated authorization or token requests, triggering excessive code challenges or userinfo fetches that degrade performance and availability.

Refresh token flows exacerbate the risk. An attacker with a short-lived access token and a valid refresh token can cyclically obtain new access tokens, driving high request volumes while appearing as legitimate authenticated traffic. ASP.NET’s token validation events and custom claims transformations can add CPU load if invoked repeatedly without rate controls. Moreover, endpoints that return user claims or scopes may expose sensitive information under rate abuse, enabling enumeration or inference attacks when responses differ by authorization state.

Effective mitigation requires integrating rate limiting with the authentication pipeline, including OpenID Connect-specific endpoints such as /connect/token, /connect/userinfo, and token revocation. Rules should consider client identifiers, scopes, and authenticated user IDs to prevent authenticated-channel abuse. ASP.NET Core provides middleware extensibility to capture these dimensions and enforce per-client and per-user ceilings without disrupting legitimate flows.

Remediation combines policy-based rate limiting with precise authentication awareness in ASP.NET Core. Use endpoint-specific limits for OpenID Connect routes and include client and subject identifiers in rate keys to prevent authenticated-channel abuse.

// Program.cs
var builder = WebApplication.CreateBuilder(args);

// Add services
builder.Services.AddAuthentication(options =>
{
    options.DefaultScheme = "Cookies";
    options.DefaultChallengeScheme = "OpenIdConnect";
})
.AddCookie("Cookies")
.AddOpenIdConnect("OpenIdConnect", options =>
{
    options.Authority = "https://auth.example.com";
    options.ClientId = "api-client";
    options.ResponseType = "code";
    options.SaveTokens = true;
    options.GetClaimsFromUserInfoEndpoint = true;
    options.TokenValidationParameters = new TokenValidationParameters
    {
        ValidateIssuer = true,
        ValidIssuer = "https://auth.example.com"
    };
});

// Configure rate limiting with policies that account for authenticated flows
builder.Services.AddRateLimiter(options =>
{
    options.GlobalLimiter = PartitionedRateLimiter.Create(context =>
    {
        // Use client ID if available, otherwise IP
        var clientId = context.User.FindFirst("client_id")?.Value ?? context.Connection.RemoteIpAddress?.ToString();
        return RateLimitPartition.GetTokenBucketLimiter(clientId, _ => new TokenBucketRateLimiterOptions
        {
            TokenLimit = 100,
            QueueProcessingOrder = QueueProcessingOrder.OldestFirst,
            QueueLimit = 20,
            ReplenishmentPeriod = TimeSpan.FromMinutes(1),
            AutoReplenishment = true
        });
    });
});

var app = builder.Build();

// Apply rate limiting before authentication to ensure abusive auth requests are throttled
app.UseRateLimiter();
app.UseAuthentication();
app.UseAuthorization();

// OpenID Connect routes with stricter limits
app.MapGroup("/connect")
    .AddRateLimiter(options =>
    {
        options.GlobalLimiter = PartitionedRateLimiter.Create(context =>
        {
            var clientId = context.User.FindFirst("client_id")?.Value ?? context.Connection.RemoteIpAddress?.ToString();
            return RateLimitPartition.GetConcurrencyLimiter(clientId, _ => new ConcurrencyLimiterOptions
            {
                PermitLimit = 20,
                QueueProcessingOrder = QueueProcessingOrder.OldestFirst,
                QueueLimit = 10
            });
        });
    })
    .MapControllers();

app.MapControllers();
app.Run();

The example demonstrates partitioning by client identifier extracted from claims, which aligns the rate limit with the authenticated client rather than only IP. This prevents authenticated users from circumventing limits via token possession. For OpenID Connect endpoints under /connect, a stricter concurrency limit reduces the risk of token and userinfo endpoint flooding.

In Startup-based configurations, ensure the UseAuthentication and UseRateLimiter ordering places rate limiting before authentication to capture abusive auth attempts. Middleware ordering matters: authentication must follow rate limiting to avoid allowing unchecked token validation requests to consume resources.

For refresh token scenarios, include a sliding window or token-idempotency key in the rate partition to detect rapid reuse. Combine with short access token lifetimes and strict audience validation to minimize the impact of token leakage. Monitor introspection and revocation call volumes per client to adjust limits based on behavioral baselines rather than static thresholds.